1. Field of the Invention
The present invention relates to a highly-steady beam generator for charged particles having a heated cathode, an auxiliary electrode and a transpierced anode.
2. Description of the Prior Art
Electron beam generating systems are already known in the art in which the beam current is controllable by an auxiliary electrode, also referred to as a Wehnelt cylinder. For various applications, for example, for material processing or electron beam microscopy or electron beam lithography, a high degree of steadiness of the electron beam is required which, however, has not yet been achieved to its full extent in practice. For example, an electron beam generating system having a heated cathode, a transpierced anode and an auxiliary anode lying at a more negative potential than the cathode has been proposed in the patent application PCT/DE/80/00086, fully incorporated herein by this reference, which is characterized in that the beam current is determined by dimensioning the active surface of the cathode and by controlling the cathode heating to a constant temperature. It is thereby achieved that the emission current is kept constant. However, the aperture angle of the beam cone changes due to thermal drifts which particularly occur in the start-up phase of operation since, when heating up the system, the spacing of the cathode to the anode is changed, whereby the shape of the electrical acceleration field is also changed. The changes are of such size that they have a disruptive effect.